The human spine contains a series of bony segments separated by discs and coupled together with muscle, ligaments, and other connective tissues. A large number of ailments may afflict one or more of these components. One exemplary ailment generally occurs with age as the spinal discs begin to break down, or degenerate resulting in the loss of fluid in the discs, and consequently, the discs become less flexible. Likewise, the discs become thinner allowing the vertebrae to move closer together. Degeneration also may result in tears or cracks in the outer layer, or annulus, of the disc. Degeneration of the annulus may allow the disc to begin to bulge outwardly. In more severe cases, the inner material of the disc, or nucleus, may extrude out of the disc. In addition to degenerative changes in the disc, the spine may undergo changes due to trauma from automobile accidents, falls, lifting, and other activities. Furthermore, in a process known as spinal stenosis, the spinal canal narrows due to excessive bone growth, thickening of tissue in the canal (such as ligament), or both. In all of these conditions, the spaces through which the spinal cord and the spinal nerve roots pass may become narrowed leading to pressure on the nerve tissue which can cause pain, numbness, weakness, or even paralysis in various parts of the body. Finally, the facet joints between adjacent vertebrae may degenerate and cause localized and/or radiating pain. All of the above conditions, as well as others not specifically mentioned, are collectively referred to herein as spine disease.
Conventionally, surgeons treat spine disease by attempting to restore the normal spacing between adjacent vertebrae. This may be sufficient to relieve pressure from affected nerve tissue. However, it is often necessary to surgically remove disc material, bone, or other tissues that impinge on the nerve tissue and/or to debride the facet joints. Most often, the restoration of vertebral spacing is accomplished by inserting a rigid spacer made of bone, metal, or plastic into the disc space between the adjacent vertebrae and allowing the vertebrae to grow together, or fuse, into a single piece of bone. The vertebrae are typically stabilized during this fusion process with the use of fusion plates and/or pedicle screws fastened to the adjacent vertebrae. The fusion plates used with the spacers are typically coupled to the spacer with a first fastening means and are positioned on the spacer such that one or more flanges extend beyond the spacer. These flanges can include one or more holes through which pedicle screws are inserted to fasten the fusion plate to the bone.
Although techniques for placing intervertebral spacers, plates, and pedicle screw fixation systems have become less invasive in recent years, they still require the placement of hardware deep within the surgical site adjacent to the spine. Extreme care needs to be taken when placing the hardware deep within the surgical site due to, for example, the close proximity of vessels and nerves to the surgical site. Of specific concern is the screws or other fastening devices used to fasten the fusion plate to the bone. If the screws are not properly aligned when driven into the bone, they may impact the nerves or vessels, which can cause severe injury to the patient. Issues with improper or incomplete placement of the hardware at the disc space can also arise if a screw is driven into the bone in a direction that results in the screw impacting a screw that has already been placed in the bone.
Thus, it would be desirable to provide a fusion plate for use with an implantable intervertebral spacer that restricted the screw trajectory variability so as to reduce or eliminate the possibility for human error when installing the fusion system.